Industrial high-speed embroidery machines generally have a workpiece support table which is mounted for movement along several axes relative to a needle carrying sewing head. The support table is driven by stepper motors which are responsive to signals from a computer control system. The signals are generated according to a digitized pattern. The workpiece is then moved under the sewing instruments through a desired path.
Typically, the sewing head includes a drive shaft to vertically reciprocate a swingable needle to penetrate a fabric to be embroidered and also to reciprocate a thread take-up lever to supply an upper thread from a supply and to tighten a stitch to be formed.
Thread breakage is a significant problem in high speed embroidering systems. It is estimated that thread breakage occurs once every few minutes in a 1000 stitch per minute machine. Effective upper thread tension control is considered important to achieving accurate stitching. If the upper thread tension is not properly controlled prior to needle penetration, thread breakage can occur. In particular, if there is too much slack in the upper thread, thread can wrap around the point of the needle, prevent loop seizure, break the thread, or interfere with correct stitch formation.
Several devices are known for controlling upper thread tension and hence for preventing thread breakage, as for example U.S. Pat. Nos. 4,320,712, 4,590,879 and 4,616,583.
Other systems for reducing thread breakage function by controlling the position of the needle thread relative to the descending needle to avoid contact between the two. For example in U.S. Pat. No. 4,706,589 to Tsukioka, a needle thread guide is disclosed for a button holing sewing machine. The needle thread guide is provided at the needle bar frame and located adjacent to the needle entry protects the needle thread from being struck by the needle when the workpiece is fed during button holing. The guide guides the needle thread outwardly when the needle descends, thus the needle thread positioned lower than the needle eye is protected from being struck by the needle. The guide is associated with the oscillating motion of the needle, but its direction of oscillation is opposite to the direction of needle oscillation, and its amplitude is almost twice the amplitude of the needle. A similar thread deflection device for zigzag stitching is disclosed in U.S. Pat. No. 4,949,657 to Hanyu, et al.
One shortcoming of these devices is that their mechanics limit their ability to effectively adapt to varying stitch and workpiece characteristics prevalent in modem high speed automated embroidery machine applications.
A need therefore exists for an improved method and system for reducing thread breakage due to the needle contacting the needle thread as it penetrates the fabric and that allows for the effective adaptation to varying stitch and workpiece characteristics and that is not limited by sewing machine mechanics.